The overall goal of this application is to test the subcellular signaling domains and specific molecular targets for calmodulin kinase II (CaMKII) in heart. Work during the previous funding interval has added significantly to a growing body of evidence showing CaMKII contributes to heart failure and arrhythmias. Studies in this proposal will take advantage of new transgenic mice generated in our laboratory with nuclear and cytoplasmic membrane-targeted CaMKII inhibition. We will also use other mice and dominant-negative constructs to genetically replace CaMKII-targeted residues on key calcium homeostatic proteins. We have evidence that challenges the prevailing paradigm that nuclear CaMKII is preeminent for regulating gene transcription. Our findings suggest CaMKII signaling in cytoplasm crosstalks to the nucleus, and we hypothesize nuclear CaMKII signaling affects excitation contraction coupling and arrhythmias. Experiments to understand if nuclear and/or cytoplasmic CaMKII actions affect for myocyte enhancer factor 2 (MEF2) regulated gene programs will use novel in vivo, cellular and molecular approaches to dissect the effects of CaMKII on MEF2 activity. The following Specific Aims will be used: 1. Test the effects of membrane-targeted and nuclear-targeted CaMKII inhibition on myocardial physiology and as protection against structural heart disease in vivo. 2. Determine the cellular and molecular targets for cytoplasmic CaMKII signaling in cardiomyocyte physiology and disease. 3. Dissect the cellular and molecular signaling pathways for beta adrenergic receptor activation of MEF2 signaling in heart. RELEVENCE. This research is directed toward solving a major public health problem. It is estimated that ~400,000 Americans die annually from sudden cardiac death. Most of these patients have a history of myocardial infarction and heart failure. Targeting these pathways with drugs may reduce suffering in these patients.